1. Field
The present invention generally relates to wireless communication systems and more particularly to secure generation of temporary mobile station identifiers.
2. Related Art
In wireless communication systems, such as Global System for Mobile Communication (“GSM”), each mobile station uses an International Mobile Subscriber Identifier (“IMSI”) to uniquely identify each user on the network. However, an attacker or eavesdropper may use a mobile station's IMSI to potentially track or eavesdrop on a mobile station user. In order to obscure the identity of the mobile user some systems, such as GSM and the Code Division Multiple Access (“CDMA”) Interim Standard 2000 (“IS-2000”), use Temporary Mobile Station Identifier (“TMSI”) in lieu of IMSI when operating in a service area.
TMSI provides security by substituting a randomly selected identification number instead of the mobile station's actual IMSI. The TMSI is selected from a large pool of possible identifiers. In GSM and CDMA IS-2000 systems the TMSI value is a thirty two (32) bit number. Thus, for GSM and CDMA the number of possible TMSI that can be assigned is 232 or 4,294,967,296.
This number provides a large number of possible TMSI assignments. The security of TMSI is derived from its large space of possible assignments that an eavesdropper must search to find a particular user. However, the total number of assignments is currently too large for current systems to manage as an assignment table in real time. Also, managing such a large table in memory requires a large amount of memory, which adds to manufacturing costs.
Some known methods for managing and assigning TMSI are as follows. One method is to select TMSIs randomly. However, a problem with a random selection is that it is possible that two or more mobile stations have the same randomly-assigned TMSI. If the network assigns about 216 TMSI, the probability that an identical assignment was made approaches one.
A second method is to assign TMSIs from a small subset of the total space to speed searches. Although this approach reduces the storage requirements, it also reduces the randomness of the TMSI. This weakens the overall strength of the TMSI protection scheme, since an eavesdropper has a better chance of predicting the next TMSI assignment, based on the current assignment.
A third method is to manage TMSI assignments as a linked list. Using a linked list minimizes the storage required. However, a linked list approach greatly increases the search time to find an entry for a TMSI or to find an unassigned TMSI for a new assignment.
Thus, it is beneficial for a wireless system to be able to practically maintain and generate secure temporary mobile station identifiers. There is therefore a need in the art for methods and systems for efficiently maintaining and securely generating TMSI assignments.